1. Field of the Invention
The present invention relates to a cable strain relief of the kind having a plurality of conductors which are embraced by a cable casing.
The invention also relates to a tool for fitting such a strain relief to a cable of corresponding diameter.
2. Description of the Related Art
A strain relief is a connection element that is coupled to the circumference of a cable and enables forces that act in the length direction of the cable, i.e. axially acting forces, to be transferred to an external construction (e.g. a cable cabinet). Such a cable will normally include a bundle of is individual, insulated conductors covered with screen braiding. An insulating sleeve is provided on top of the braiding.
Such strain reliefs will preferably be electromagnetically impervious, i.e. provide electromagnetic insulation (EMI), and must also be capable of transferring forces effectively, so as to prevent individual fibers from being subjected to tensile forces in junction boxes or the like.
A known type of strain relief intended for this purpose is comprised of two tubular parts that have co-acting conical surfaces. One part (the inner part) has slits that extend axially from one end thereof, such as to form axially extending tongues. When the two parts are fitted together axially, the free ends of the tongues will be bent inwardly against the outside of the cable. The outer insulation is removed at the end of the cable and the braiding is folded back around the end of the remaining insulating sleeve. Consequently, when the two tubular parts are fitted together, parts of the braiding threads will be clamped between adjacent tongues. This often results in unsatisfactory anchoring of the strain relief to the cable.
Other known strain reliefs incorporate a U-shaped element whose legs are bent in towards the cable so as to overlap each other. This solution can result in damage to the individual conductors in the cable, and may also cause EMI-leakages to occur through the overlap.
Earlier known strain reliefs are expensive and require the use of complex devices for fitting the reliefs to cables, and also result in joints of greatly differing qualities.
Accordingly, the object of the invention is to provide a strain relief that can be fitted to cables of different standard diameters with the aid of a simple tool, such as to obtain an EMI-tight connection and a uniform result on each occasion.
Further objects of the invention will be evident from the following text, either directly or indirectly.
The invention is basically concerned with establishing a strain relief for a screened cable that includes a plurality of conductors, by crimping a crimp sleeve on that part of the cable at which the braiding has been folded back over said cable, said crimp sleeve incorporating means which function to achieve an EMI-tight coupling of the sleeve to the wall of an apparatus housing at the cable leadthrough or transit. Crimp sleeves of this kind are available in various diameters, lengths and wall thicknesses adapted to different cable diameters, so as to ensure that the crimp sleeve will be effectively anchored to the cable with the aid of a crimping tool and therewith provide an EMI-impervious cable connection.
In order to enable a generally uniform crimping force to be applied with sleeves of different diameters and different wall thicknesses, the larger crimp sleeves include a circumferential groove that reduces the crimping force required to achieve the requisite strain relief anchorage of the crimp sleeve to the cable.
The inventive crimping tool for crimping such strain relief sleeves may include a tool frame structure that has two generally parallel and mutually opposing frame members. Two opposing crimp inserts are placed between the frame members. The inserts include on their mutually facing sides recesses for crimping a sleeve of corresponding diameter on a corresponding cable. A screw meshes with a threaded hole through one frame member, so that the screw can be screwed in a direction towards the second frame member such as to bring the two inserts together. The inserts include means for guiding relative movement in said direction. The two inserts also include spring means which strive to move the inserts apart in said direction.
The two frame members have shallow recesses for receiving and localizing respective inserts. The tools can be used in conjunction with a set of insert pairs which each include crimping recesses that are adapted to crimp sleeves of correspondingly different sizes. In other respects, the insert pairs have generally equal outer dimensions in order to enable the insert pairs to be readily swapped in the frame structure. Owing to the relative guiding of the inserts and the spring means, the pair of inserts will be held automatically in the frame structure as soon as the inserts are inserted thereinto, and can readily be replaced manually with another pair of inserts, by first pressing the fitted pair of inserts together against the action of the spring means and then tipping the inserts out of one of the recesses and out of the frame structure. The recesses in the frame members are shallow recesses. The frame structure may conveniently have an elongated support arm and the screw may include a lever for facilitating rotation of the screw.